Sun is the shining spherical heavenly body around which the planets rotate. It is one of some 10,000,000 stars which constitute our galaxy. Its average distance from Earth is 149,597,900 Km., equatorial diameter 1,392,520 Km., mass 2*10²⁷ tons and its average density 1.4 grams per cc.

Planets revolve round the Sun. They are not self-luminous but shine by radiating the light received from the Sun. Their orbits are elliptical. Their sizes, speeds and distances from the Sun are dissimilar. There are nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and  Pluto. Pluto is farthest from the Sun and Mercury nearest.

Satellites are secondary bodies that revolve around the planets as planets revolve around the Sun. A satellite is said to have been formed of the matter whirled off from the planet when still in molten state. Except Venus and Mercury, all the other planets have satellites. Moon is Earth's satellite.

Asteroids are minor planets whose orbits lie between Jupiter and Mars. These are said to be the fragments of a larger planet disrupted long ago. Their number is estimated to be 30,000 pieces of rocky debris out of which 18,000 are known. Ceres, the first to be discovered is the largest asteroid having a diameter of 670 Km. most of them are less than 80 Km. in diameter.

Meteors are small bodies coming from interplanetary space. The become luminous by friction on entering the Earth's atmosphere and are popularly known as shooting stars.

Meteorites are larger meteors that reach the Earth and become Meteorites. All Meteorites were meteors when in flight.

Stars are suns or self-luminous bodies, situated at enormous distances from the solar system. Some of the stars are so distant it takes millions of years for their light to reach us. The distances of stars are expressed in light years. There are millions of stars in the universe. There are four kinds of stars:

Red Giants are stars which have consumed about 10% of their Hydrogen on account of which they appear reddish. red giants consume their hydrogen at an increasing rate and eventually contract to become White dwarfs.

Comet is luminous celestial body which moves about the solar system in elliptical or hyperbolic orbits. Comets are usually accompanied by a long shining tail. Hyperbolic comets are seen only once and the they do not reappear. Elliptical comets are periodic and their recurrence can be calculated, as in the case of Halley's comet.

Pulsars are highly compact stellar objects, distinct from the stars one sees in the night sky, rotating rapidly and emitting electromagnetic radiation in pulses much in the manner of a light house flashing light. Pulsars are thought to be rapidly spinning neutron stars, in which matter is in the form of 'degenerate' neutron liquid with densities reaching values of 100 million million gm per cc. Astronomers expect the newly born neutron stars to be pulsars. 

Cosmos...